1. Field of the Invention
This invention relates to the processing of audio signal data. More specifically, the invention provides a system and method for intelligently synthesizing audio data to conceal errors detected in a received audio signal.
2. Description of the Related Art
Growing numbers of high-quality digital audio reproduction systems have heightened the demand for the transmission of digital audio data. Much of that demand is based on a desire to hear a live playback of an audio selection, such as music or broadcasts of news or sporting events.
Digital audio broadcast systems now exist which are capable of streaming digital audio data to audio receiving systems for immediate playback. Most communication networks, however, cannot guarantee that all audio information that is transmitted by an audio transmission system will be received error-free by all receiving systems.
One example of such a communication network is the Internet. Audio data streaming systems now exist which transmit audio data in packets over the Internet, with the packets being received by audio playing applications for immediate and continuous playback. While the Internet is reasonably reliable for successfully transmitting data from a sending system to a receiving system, the transmission is not necessarily guaranteed. In the case of UDP protocol transmission, the packets may arrive out of order, late or not at all. Connections, such as UDP connections, routinely drop or lose packets. Audio data packets are no exception.
Some attempts have been made to allow audio receiving systems to conceal the effects of lost audio packets. Early techniques merely muted lost packets, that is, substituted silence for lost audio data. Other techniques simply replicate the last successfully received packet to take the place of a lost packet. This results in the unpleasant experience of the same sequence of audio information being played twice, or sometimes over and over again in the case when a series of audio packets is lost.
An improved, but still dissatisfactory technique is disclosed in U.S. Pat. No. 5,673,363 to Jeon et al. for an Error Concealment Method and Apparatus of Audio Signals. That patent discloses a technique of reconstructing a frame of lost audio information by applying predetermined weight values to frequency coefficients of adjacent frames which do not have errors. The problem with that technique and other existing techniques is that it ignores important signal characteristics surrounding the lost audio data. For example, the technique will simply use the frequency coefficients of a neighboring frame to reconstruct a lost frame, even though those frequency coefficients may represent a sharp change or attack in an audio signal, with the result being an extremely unpleasant and disruptive repeat of an audio attack during playback.
There is now a tremendous need for a system and method capable of discriminating among signal characteristics used to reconstruct lost audio data.
One embodiment of the present invention is a method for creating audio signal data representing audio data lost during a transmission. The method comprises the steps: (1) receiving first audio data from an audio transmission; (2) receiving second audio data from an audio transmission; (3) detecting the loss of audio data between said first and second audio data; (4) determining the presence of a transient audio signal in said first audio data; (5) decoding said second audio data to create second frequency domain data; and (6) interpolating synthetic frequency domain data by applying an interpolation weight to samples in said second frequency domain data. In a preferred aspect, the method comprises the further step of decoding said synthetic frequency domain data to generate time domain data for audio reproduction. In another aspect, the method comprises determining the presence of a transient audio signal in said second audio data; decoding said first audio data to create first frequency domain data; and nterpolating synthetic frequency domain data by applying an interpolation weight to samples in said first and second frequency domain data.
In another embodiment, the present invention is a system for concealing errors during audio playback caused by lost audio data. The system comprises: (1) a buffer storing first and second audio data; (2) an audio loss detector detecting an absence of audio data expected between said first and second audio data; (3) an audio decoder generating second frequency domain data from said second audio data; (4) a transient detector for detecting the presence of a transient audio signal in said first audio data; and (5) a frame synthesizer interpolating synthetic audio data to fill said absence by applying an interpolation weight to said second frequency domain data.
In a further embodiment, the present invention is a system for concealing errors caused by lost audio data in an audio transmission. The system comprises (1) means for receiving audio data; (2) means for detecting lost audio data; (3) means for decoding received audio data to generate frequency domain data; (4) means for detecting transient audio signals in received audio data; and (5) means for synthesizing audio frame data from frequency domain data.